Implantable stimulation devices generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, occipital nerve stimulators to treat migraine headaches, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention in a microstimulator device system of the type disclosed in U.S. Patent Publ. No. 2010/0268309, which is incorporated herein by reference in its entirety. Despite being described in the context of a microstimulator device system, the invention can also be used in any implantable stimulator device system, such as a Spinal Cord Stimulator (SCS) used to treat lower back pain, such as is disclosed in U.S. Pat. No. 7,444,181, which is incorporated herein by reference in its entirety, for example. “Microstimulator” as used in the following should thus be understood as comprising any implantable stimulator.
Microstimulator devices typically comprise a small, generally-cylindrical housing which carries electrodes for producing a desired stimulation current. Devices of this type are implanted proximate to the target tissue to allow the stimulation current to stimulate the target tissue to provide therapy for a wide variety of conditions and disorders. A microstimulator usually includes or carries stimulating electrodes intended to contact the patient's tissue, but may also have electrodes coupled to the body of the device via a lead or leads. A microstimulator typically has two electrodes, although microstimulators can also have more than two electrodes in an array, such as is disclosed in U.S. Pat. No. 7,881,803, which is incorporated herein by reference in its entirety, for example. Microstimulators benefit from simplicity. Because of their small size, the microstimulator can be directly implanted at a site requiring patient therapy.
FIG. 1 illustrates in cross-section an exemplary implantable microstimulator 10 having only two electrodes 12a and 12b. As shown, the microstimulator 10 includes a power source 14 such as a battery, control circuitry (e.g., a microcontroller) 16, and various electrical circuitry 20 including stimulation circuitry for forming stimulation pulses at the electrodes 12a/b, and a coil 22. Stimulation pulses may be defined by a stimulation program (SP) stored in memory, which memory may be associated with either or both of the microcontroller 16 and the electrical circuitry 20. A stimulation program may define the amplitude, pulse width, and frequency of the pulses, or other parameters of the pulses, as explained further below.
Electrical components are integrated by a circuit board 24 and housed within a capsule 26, which is usually a thin, elongated cylinder, but may also be any other shape as determined by the structure of the desired target tissue 5, the method of implantation, and/or the number and arrangement of external electrodes 12a/b. 
The battery 14 supplies power to the various components within the microstimulator 10, including power for providing the stimulation current sourced or sunk from the electrodes 12a/b as provided by circuitry 20. Battery 14 may be a primary battery, a rechargeable battery, a capacitor, or any other suitable power source.
The coil 22 is configured to receive and/or emit a magnetic field that is used to communicate with, and/or receive power from, one or more external devices that support the implanted microstimulator 10, examples of which will be described below. Such communication and/or power transfer may be transcutaneous (i.e., through a patient's tissue 5) as is well known. Transmitter/receiver circuitry may be coupled to coil 22, as explained further below.
The illustrated microstimulator 10 includes electrodes 12a/b on the exterior of the capsule 26. The electrodes 12a/b may be disposed at either end of the capsule 26 as illustrated, or placed along the length of the capsule. There may also be more than two electrodes arranged in an array, as described earlier. One of the electrodes 12a/b may be designated as a stimulating electrode, with the other acting as an indifferent electrode (reference node) used to complete a stimulation circuit, producing monopolar stimulation. Or, one electrode 12a/b may act as an anode while the other acts as a cathode, producing bipolar stimulation. Electrodes 12a/b may alternatively be located at the ends of short, flexible leads. The use of such leads permits, among other things, electrical stimulation to be directed to targeted tissue(s) a short distance from the surgical fixation of the bulk of the microstimulator 10. In one example, microstimulator 10 may be built as disclosed in U.S. Pat. No. 7,351,921, which is incorporated herein by reference in its entirety.
Turning to FIGS. 2A and 2B, the microstimulator 10 is illustrated as implanted in a patient's tissue 5, and further shown are various external components that may be used to support the implanted microstimulator 10. An external controller 30 may be used to control and monitor the microstimulator 10 via a bidirectional communication link 35. Communication on link 35 can occur via magnetic inductive coupling between the external controller's coil 32 and the microstimulator's coil 22 as is well known. Typically, the magnetic field on link 35 is modulated, for example with Frequency Shift Keying (FSK) modulation or the like, to encode transmitted data. For example, data telemetry via FSK can occur around a center frequency of fc=125 kHz, with a 129 kHz signal representing transmission of a logic ‘1’ bit and 121 kHz representing a logic ‘0’ bit.
The external controller 30 is generally similar to a cell phone for example and includes control circuitry (e.g., a microcontroller) 34, a battery 36, and a port such as a USB port 38 which is formed in the controller's hand-holdable and portable housing 40. The external controller 30 can include a user interface including buttons 42 and a display 44, and may include other user interface elements such as a speaker (not shown). The various electronic components may be integrated in the external controller 30 using a circuit board 46.
An external charger 50 provides power to recharge the microstimulator's battery 14 (FIG. 1). Such power transfer occurs by energizing a coil 52 in the external charger 50, which produces a magnetic field comprising link 55, which may occur with a different frequency (f2=80 kHz) than data communications on link 35. This magnetic field 55 energizes the coil 22 in the microstimulator 10, which is rectified, filtered, and used to recharge the battery 14, as explained further subsequently. Link 55, like link 35, can be bidirectional to allow the microstimulator 10 to report status information back to the external charger 50, again as explained subsequently. For example, once control circuitry 16 in the microstimulator 10 detects that the battery 14 is fully charged, its coil 22 can signal that fact back to the external charger 50 so that charging can cease.
The external charger 50 generally comprises a hand-holdable and portable housing 54, in which are contained a battery 56 for powering the charger's electronics, including circuitry 58, which may include a microcontroller 58 for example. The external charger 50's circuitry may be integrated on one or more circuit boards 60, as explained for example in U.S. Pat. No. 9,002,445, which is incorporated herein by reference in its entirety. The external charger 50 may have a relatively simple user interface, including for example only an on/off button 62 to begin production of the magnetic field comprising link 55, and may additionally include an indicator, such as a Light Emitting Diode (LED) 64 or a speaker (not shown). Although not depicted, the external charger 50 may include a display as well.
In other examples, data communication and charging functionality may be integrated in a single external device or system. For example, and although not illustrated, data communication and charging may be integrated within a single housing, as disclosed in U.S. Pat. No. 8,335,569, which is incorporated herein by reference in its entirety. Alternatively, the charging coil 52 can comprise an assembly coupleable by a cable to port on the housing of the controller, which controller can comprise the external controller 30 as disclosed in U.S. Pat. No. 8,498,716, which is incorporated herein by reference in its entirety, or which controller is specifically dedicated to charging functionality without implicating implant data communications. In either case, integration of the external charger with an external communicator generally allows charging functionality to benefit from the external controller's provision of an improved user interface, in particular its display.
A further external device supporting the microstimulator 10 is shown in FIG. 2B, which comprises a well-known clinician programmer 70, and which may be as described in U.S. Patent Application Publication 2015/0360038, which is incorporated herein by reference in its entirety. A clinician programmer 70 is generally used by a clinician to control and monitor a patient's microstimulator 10 in a clinical setting. For example, clinician programmer 70 can be used after implantation to initially program the microstimulator 10 with a stimulation program that is most effective for the patient, although the patient may later modify this program in certain respect using his external controller 30. The clinician programmer 70 may also be used for routine check-ups to adjust the stimulation program or monitor microstimulator 10 operation.
The clinician programmer 70 typically comprises a personal computer 72, which may be portable, such as a laptop or tablet computer. The computer 72 includes a display 74 with a graphical user interface 80 rendered by clinician programmer (CP) software 78 executed by control circuitry 76 of the computer 72. As the computer 72 may not inherently have means to communicate directly with the implant, the clinician programmer 70 can include a communication head 82, sometime called a “wand.” The communication head 82 includes an antenna coil 88 similar in function to the coil 32 in the external controller 30, and capable of communicating with the coil 22 in the microstimulator via link 95 (e.g., by FSK). The communication head 82 is coupled to a port 86 of the computer 72, which may comprise a USB port for example. If necessary, the communication head 82 may also include modulation and demodulation circuitry, although not shown.
Due to its small size, a microstimulator such as 10 is useful in providing neurostimulation in many locations within the human body and for many different therapeutic purposes, such as those already mentioned. However, the inventor notes that due to its relatively small size, the battery 14 within the microstimulator 10 is also necessarily small, and hence of low capacity. For example, battery 14 may only have a capacity of 20 mAh for example. Depending on the therapy the microstimulator must provide, a battery 14 of this capacity may not allow the microstimulator to operate for a sufficiently long period of time between charging sessions provided by the external charger 50. This disclosure addresses this problem by providing a system in which external power is provided differently for the microstimulator 10 depending on the therapy the microstimulator is providing.